The present invention relates to a semiconductor memory device and a method for manufacturing the same.
In conventional nonvolatile semiconductor memory devices, a place between word lines thereof is filled in with an oxide or nitride film, the word line having a tunnel oxide film, a floating gate electrode, an interpoly insulating film, and a controlling gate electrode which are stacked in order. However, an interval between word lines is shorten as elements are miniaturized, and therefore the conventional devices have problems as follows: increased variations in a threshold voltage of its floating gate due to a parasitic capacitance produced between the floating gate electrodes of adjacent word lines thereof; and reduced writing speed due to parasitic capacitances produced between the adjacent word lines and between the floating gate and diffusion layer. Moreover, it has a problem that a high electric field applied between the electrodes destroys a buried material between the electrodes.
In order to solve such problems, it has been proposed to provide an air gap (a cavity) between word lines. For example, known is a method for forming an air gap by conducting an ashing process to remove a sacrificial film with organic materials as a sacrificial film (e.g. see Japanese Patent Laid-Open No. 01-137651). However, the method has a problem that, when such a method is applied to formation of an air gap between word lines, carbons and the like contained in organic materials causes the tunnel oxide film to be deteriorated.
Also, known is a method for stacking an oxide film with poor filling-in characteristics on word lines and between the word lines and then providing a void between adjacent floating gate electrodes (e.g. see U.S. Patent Application Publication No. 2006/0001073). However, such a method has problems that variations in position and shape of the voids occur as well as variations in a threshold voltage for each cell occur, thereby decreasing reliability thereof.
Thus, the conventional method for forming an air gap has the problems of decreasing reliability of a semiconductor memory device.